Male sexual behaviour, including arousal, courtship and the act of copulation, has long been thought as solely the domain of the brain.
The finale – ejaculation – is a reflex controlled by the spinal cord.
But new research in mice and rats suggests that the spinal cord plays a more important role in sex and arousal than ever realised. The findings, published in a study in Nature Communications, have implications for studying human sexuality and premature ejaculation.
“The spinal cord isn’t just a passive relay station executing brain commands”, says Susana Lima, senior author and principal investigator of the Champalimaud Foundation’s Neuroethology Lab, Portugal.
“It integrates sensory inputs, responds to arousal, and adjusts its output based on the animal’s internal state. It’s much more sophisticated than we imagined”.
To start, the researchers wanted to know which neurons control the muscle responsible for ejaculation – the bulbospongiosus (BSM).
“It sits just below the penis and is critical for sperm expulsion,” explains Constanze Lenschow, co-lead author from the Institute for Cognitive and Integrative Neuroscience of Aquitaine at the University of Bordeaux, France.
“When a male ejaculates, the BSM fires in a characteristic burst pattern. It’s like the signature of ejaculation”.
In mice, they traced the pathway from the BSM to the motor neurons which give the command for it to contract.
In previous research, the team had identified that spinal cord neurons expressing a molecule called galanin (Gal) were key to ejaculation. Now, they genetically modified mice so that Gal-expressing (Gal+) neurons fluoresced red. Under the microscope, they found that the axons of these neurons overlapped with the BSM motor neurons.
“When we activated the far ends of Gal⁺ neurons – where they pass on signals – we recorded a burst of activity in the BSM motor neurons,” says Lenschow.
“And when we blocked glutamate – the chemical these neurons use to excite others – the signal disappeared, confirming a direct, excitatory connection.”
“Interestingly,” Mendes adds, “Gal⁺ neurons didn’t just project to the ejaculation muscle, they also connected to other areas involved in erection and the autonomic control of ejaculation”.
Experiments showed that, in mice, Gal⁺ neurons received sensory input from the penis.
But, while stimulating these neurons reliably triggered ejaculation in rats, the process was more complicated in mice.
“We could get the BSM to fire, but stimulation of Gal⁺ neurons never led to a real ejaculation,” says Lenschow.
“And unlike in rats, when we repeated Gal⁺ stimulation, BSM responses weakened. It was as if the system had entered a refractory state after that initial activation.”
This effect disappeared when the mice’s brain activity was removed from the equation.
Mendes says, “Our findings support a model where descending input – likely from a brainstem region – inhibits the Gal⁺ neurons and incoming genital signals until the animal reaches the ejaculatory threshold.”
“If the mouse had already ejaculated, Gal⁺ stimulation didn’t work – the BSM just wouldn’t respond,” adds Lenschow.
“That told us these neurons weren’t just coordinating ejaculation. They were integrating the animal’s internal state”.
Destroying these cells with a targeted toxin completely blocked ejaculation in rats but left their mating behaviours unchanged.
“In mice, however, the effect was more subtle,” says Mendes.
“Only 3 out of 12 males failed to ejaculate, and many showed a disrupted sequence: they struggled to find the vagina, and took longer to ejaculate, with more failed mounts and probing attempts.”
This suggests that ejaculation is more of a reflex in rats, while in mice Gal+ neurons receive sensory input, modulate motor output, and integrating signals related to arousal and internal state.
“Rats may be good models for studying premature ejaculation,” notes Lenschow, “but mice might actually be better for understanding how human sexuality works – how arousal builds, and how ejaculation is regulated.”
Next, the team plans to record Gal⁺ neurons during sex to understand how their firing patterns interact with other organs like the brain and prostate.